Oil sand generally comprises water-wet sand grains held together by a matrix of viscous heavy oil or bitumen. Bitumen is a complex and viscous mixture of large or heavy hydrocarbon molecules. The Athabasca oil sand deposits may be efficiently extracted by surface mining which involves shovel-and-truck operations. The mined oil sand is trucked to crushing stations for size reduction, and fed into slurry preparation units where hot water and caustic (sodium hydroxide) are added to form an oil sand slurry. The oil sand slurry may be further conditioned by transporting it using a hydrotransport pipeline to a primary separation vessel (PSV) where the conditioned slurry is allowed to separate under quiescent conditions for a prescribed retention period into a top layer of bitumen froth, a middle layer of middlings (i.e., warm water, fines, residual bitumen), and a bottom layer of coarse tailings (i.e., warm water, coarse solids, residual bitumen). The bitumen froth, middlings and tailings are separately withdrawn. The bitumen froth is de-aerated, heated, and treated to produce diluted bitumen which is further processed to produce synthetic crude oil and other valuable commodities.
“Fines” are particles such as fine quartz and other heavy minerals, colloidal clay or silt generally having any dimension less than about 44 μm. “Coarse solids” are solids generally having any dimension greater than about 44 μm. Oil sand extraction typically involves processing ores which are relatively high in bitumen content and low in fines content. However, there exists an abundance of “poor ores”, also referred to as “poor processing ores”, which alone yield poor bitumen recovery and consequently cannot be processed unless a high proportion of high-grade, good ores are blended into these dry ore feeds. “Poor ores” are oil sand ores generally having low bitumen content (about 6 to about 10%) and/or high fines content (greater than about 30%). In comparison, “good ores” or “good processing ores” are oil sand ores generally having high bitumen content (about 10 to about 12% or higher) and/or low fines content (less than about 20%).
Caustic is used in bitumen extraction to improve bitumen recovery and froth quality. Caustic promotes the release of natural surfactants from bitumen to the aqueous phase, precipitates divalent cations such as calcium and magnesium, modifies the electrical surface potential of bitumen and solids, adjusts the pH, and makes solids more hydrophilic, leading to better bitumen-solids separation. For an oil sand ore, there is normally an optimal caustic dosage at which the highest bitumen recovery can be obtained and the optimal dosage appears to be determined by both the fines content (Sanford, E., 1983, Can. J. Chem. Eng. 61:554-567) and the ore grade.
However, the use of caustic creates undesired consequences. Caustic is toxic and corrosive, impacting health and the environment and causing scaling on equipment due to precipitation of divalent cations when it is added to the slurry water for slurry preparation. Compared to the caustic dosage for good ores, a higher caustic dosage is required for poor ores, but does not necessarily improve bitumen recovery and froth quality. Poor ore feed often results in high PSV middlings' density and viscosity, leading to low recovery and poor bitumen froth quality. The current solution is to reduce the feed rate and to add more water at the price of lowering production. However, for some poor ores, the use of caustic alone does not provide sufficient improvement in processability. Caustic disperses fines, hindering fines settling and tailings treatment. Higher caustic dosages induce bitumen emulsification which impairs froth treatment.
Due to these problems, it is desirable to replace caustic with an alternative chemical, or to reduce the amount of caustic used in the extraction process. A great number of chemicals have been tested as an alternative for caustic, but were not as effective and economic as caustic.
Accordingly, there is a need for a method of minimizing the amount of caustic used in bitumen extraction while improving overall extraction performance, especially for poor processing ores.